This invention relates to thick film circuits and in particular to the fabrication of such circuits which include a non-noble metal conductor for interconnection of the circuit elements.
Most thick film circuits presently employ a conductor system which is based on the noble metals silver, gold, palladium and platinum, alone or in combination, and a resistor material including a ruthenium-based composition. Although such circuits perform satisfactorily, several problems are involved in the use of precious metal pastes. A major problem is the rising cost of such precious metals, which has a significant impact on the economies of thick film circuit fabrication. In addition, silver in the paste tends to migrate across the substrate surface and/or through dielectrics to cause short circuits. Furthermore, conductor pastes including silver, gold or palladium can cause poor solderability of the circuit.
To solve such problems, those skilled in the art have sought alternatives to the noble metal pastes. One such alternative involves use of a copper-based conductor. Copper thick film pastes which include glass frits are commercially available, but they are normally fired in a nitrogen atmosphere which is incompatible with ruthenium-based resistor material (see, e.g., U.S. Pat. No. 4,072,771, issued to Grier, Sr.). Copper pastes without glass frits generally require a high temperature firing in an oxidizing atmosphere followed by a high temperature firing in a reducing atmosphere which causes severe degradation of resistor materials requiring an oxidizing atmosphere.
A recent proposal provided a technique for making a fritless copper paste conductor requiring a reducing atmosphere compatible with a resistor material requiring an oxidizing atmosphere (see, U.S. Pat. No. 4,140,817, issued to Brown). In that process, after an initial firing in air, the conductor material was subject to a high temperature reduction followed by a re-oxidation to produce a copper oxide having a lower density than the oxide formed after the initial firing. The resistor material was then deposited and fired in an oxidizing atmosphere, and the low density copper oxide was reduced at a low temperature so resistor properties were not affected. While the process presented a viable solution to the problem, it suffered from the disadvantages of requiring a high initial firing temperature in a special furnace, as well as extra processing steps.
It is therefore a primary object of the invention to provide a method for fabricating a thick film circuit including non-noble metal conductors whereby the formation of the conductors is compatible with the formation of resistors requiring an oxidizing atmosphere. It is a further object of the invention to provide such a method with a minimum of processing steps and low firing temperatures.